The present invention relates generally to an active clamping circuit for power amplifiers and, more specifically, to an active clamping circuit for power amplifiers that reduces the gain of the amplifier when its output voltage becomes dangerously high, protecting the amplifier from damage.
Power amplifiers such as those included in mobile phones may be subjected to extreme operating conditions which can lead to problems including amplifier failure. For example, if the impedance of the mobile phone antenna is mismatched to that of the amplifier circuit, Voltage Stand Wave Ratio (VSWR) and, thus, the voltage level at the output of the amplifier can increase to a dangerous level causing power amplifier failure and rendering the mobile phone useless.
To prevent amplifier failure, active control circuits may be added to the power amplifier that can adjust its gain to keep it within safe operating conditions. There are control circuits in the prior art that can control the gain of power amplifiers. However, many of them involve complex circuits which are expensive and take up valuable space on the integrated circuit or printed circuit board. In addition, some prior art control circuits target voltage breakdown-causing events occurring at the input of the power amplifier rather than at its output. Other prior art active control circuits are only geared towards improving the operating condition of the power amplifier, such as increasing its operational bandwidth or preventing it from reaching saturation rather than preventing power amplifier failure.
Therefore, there is a need for an improved active control circuit that is simple and compact and can prevent events occurring at the output of a power amplifier from causing its failure.
It is therefore an object of the present invention to provide an active clamping circuit that is simple and compact and can prevent failure of the amplifier.
It is another object of the present invention to provide an active clamping circuit that can prevent amplifier failure caused by signals transmitted from external sources to the amplifier output.
Briefly, the present invention provides an active clamping circuit that reduces the gain of a power amplifier when high voltages are present at its output stage. In a preferred embodiment, the active clamping circuit comprises a transistor with its base connected to a diode stack which is in turn connected to the output of the power amplifier. The diode stack may be of the type used previously for overvoltage protection in such devices and functions as a triggering circuit to trigger the active clamping circuit in overvoltage situations. The collector of the transistor is connected to a biasing circuit of the power amplifier that controls the latter""s gain. During normal amplifier operation, the diode stack does not conduct and the active clamping circuit is off. However, when a high voltage signal is present at the output of the power amplifier, the diode stack conducts, switching on the active clamping circuit. The active clamping circuit, now switched on, reduces the bias level of the power amplifier, lowering its gain until the voltage level at the amplifier output is brought to a safe level. Optionally, an integrating filter is connected to the base of the transistor to fine tune the threshold level at which the clamping circuit becomes active.